This invention relates generally to an apparatus and method for controlling an automotive generator, and more specifically to an apparatus and method for selectably choosing a point for sensing the output voltage from the generator.
Voltage regulator controlled automotive battery charging systems are commonly used to keep an automotive storage battery at full charge level and to provide sufficient electrical power for the operation of the various automobile electrical accessories. In a conventional motor vehicle, the electrical energy is generated by a generator driven by means of a belt-and-pulley arrangement coupled to the internal combustion engine of the automobile. The generator includes a field winding, a three-phase stator winding, and a full-wave diode bridge rectifier for converting the polyphase AC output of the windings to a DC output voltage for supplying various vehicle loads, including the vehicle battery. The voltage regulator associated with the generator controls the field coil excitation by controlling the current supplied (from a current source) to the field winding, to thereby control the DC output voltage to a desired value, which in turn ensures that the battery voltage is maintained at the desired level.
This control of the DC output voltage is effectuated by a feedback loop for measuring the generator output voltage or the actual battery voltage, and based on this value, controlling the field excitation current supplied to the generator field winding to bring the DC output voltage to the desired magnitude. Since the objective of the automobile charging system is to maintain the battery voltage at a predetermined value, it is preferable to measure the sensed battery voltage, rather than the generator output voltage, and use the sensed value to control the field current excitation. The generator output voltage can be measured at the DC output terminal of the generator and the battery voltage can be sensed at the battery, or at another point in the vehicle""s electrical system proximate the battery. If the latter technique is used, so called external sensing, it is critical that the externally sensed voltage accurately represent the battery voltage. If the external sense voltage point develops a high resistance due to corrosion or grime, or if unwanted resistances appear in the conductor between the external sense point and the voltage regulator terminal, the voltage drop caused thereby provides a false reading as to the actual battery voltage. In turn, the false voltage reading signals the generator to unnecessarily increase the DC voltage output. In situations where the generator output voltage is increased above its nominal value, the generator and the other vehicle components to which it supplies a voltage can be damaged. In certain especially egregious situations, the high resistance and attendant voltage drop can raise the alternator output voltage to 18 to 21 volts. Generally, the output voltage from the alternator should never exceed 16 volts. Above this level, fuses may blow and other vehicle components may be damaged. Further, output voltages at this level may ruin the battery by boiling it dry.
The disadvantages of the prior art technique wherein the feedback loop can cause the generator output voltage to exceed a nominal value due to a false measurement of the battery voltage, caused by unexpected resistances in the circuit to an external voltage sensing point, are overcome by the present invention. The apparatus and method of the present invention determines whether to use the generator output voltage (internal sense) or the battery voltage at an external sense point to control the field excitation current to the alternator.
The present invention senses the voltage at both the generator output terminal (internal sense) and the external terminal and compares these two values. If the external sense value is less than a predetermined absolute threshold low voltage, then a failsoft condition is detected and the voltage regulator begins sensing from the internal sensed point. Also, if the external sense voltage is less than the difference between the internal sensed voltage and a predetermined offset threshold low voltage, then a failsoft condition is also detected. If the external sensed point voltage is greater than a predetermined absolute threshold high voltage and greater than the difference between the internal sensed terminal voltage and a predetermined offset high threshold value then sensing from the external sense point is permitted.
The voltage regulator operates in the conventional manner when sensing voltage from the generator output terminal (internal sense). That is, if the generator output voltage at the sensed point exceeds a predetermined value, the voltage regulator changes the duty cycle of the current supplied to the field winding to a minimum value to decrease the generator output voltage. If the voltage at the generator output terminal goes above a second output voltage threshold, the regulator switches off the field excitation current. If the generator output voltage remains above the second output threshold value for longer than a predetermined time, then the regulator provides a fault indication to the vehicle operator.
The voltage regulator of the present invention also includes a failsafe condition that exists when the voltage regulator is sensing from the external terminal and the internal sense point exceeds a failsafe threshold voltage. Under these conditions, the voltage regulator uses the internal sense point and latches in a failsafe mode.